Coconuts and monkeys?
There are 5 sailors that crash on an island. This island also has a monkey on it.
On the day the sailors arrived at the island, they started to collect as many coconuts as they can. After dusk, they slacked their coconuts in a big pile and went off to sleep. One of the sailors woke up during the night and split the big pile of coconuts into 5 plies, with one remainder. He gave that one coconut to the monkey and took one of the 5 piles and hid them. The 4 other piles he stacked back up together so that the other sailors don't notice anything. The four other sailors do the exact same thing that night. In the morning the pile has significantly reduced and the 5 sailors split the pile of coconuts 5 ways exactly (this time the monkey doesn't get anything). How many coconuts were there originally?